useful for the technical students

2. CONTENT
1.Introduction
2.Single line diagram of AC power
transmission system
3.Different types of transmission
lines
4.Main elements of transmission line

3. INTRODUCTION
Electrical energy, after being produced at generating
stations (TPS, HPS, NPS, etc.) is transmitted to the
consumers for utilization.
This is due to the fact that generating stations are usually
situated away from the load centers. The network that
transmits and delivers power from the producers to the
consumers is called the transmission system. This energy
can be transmitted in AC or DC form. Traditionally, AC has
been used for years now, but HVDC (High Voltage DC) is
rapidly gaining popularity

4. SINGLE LINE DIAGRAM OF AC POWER TRANSMISSION
SYSTEM

5. Electric power is commonly (or usually) generated at 11 kV in
generating stations in India and Europe.
While in some cases, generation voltage might be higher or
lower. Generating machines, to be used in power stations, are
available between 6 kV to 25 kV from some big
manufacturers.
This generating voltage is then stepped up to 132kV, 220kV,
400kV or 765kV etc. Stepping up the voltage level depends
upon the distance at which power is to be transmitted.
Longer the distance, higher will be the voltage level.
Stepping up of voltage is to reduce the I2R losses
in transmitting the power (when voltage is stepped up, the
current reduces by a relative amount so that the power
remains constant, and hence I2R loss also reduces). This
stage is called as primary transmission.

6. The voltage is the stepped down at a receiving station to 33kV or
66kV. Secondary transmission lines emerge from this receiving
station to connect substations located near load centers (cities
etc.).
The voltage is stepped down again to 11kV at a substation. Large
industrial consumers can be supplied at 11kV directly from these
substations. Also, feeders emerge from these substations. This
stage is called as primary distribution.
Feeders are either overhead lines or underground cables which
carry power close to the load points (end consumers) up to a
couple of kilometers. Finally, the voltage is stepped down to 415
volts by a pole-mounted distribution transformer and delivered to
the distributors. End consumers are supplied through a service
mains line from distributors. The secondary distribution system
consists of feeders, distributors and service mains.

7. DIFFERENT TYPES OF TRANSMISSION SYSTEMS
Single phase AC system
 single phase, two wires
 single phase, two wires with midpoint earthed
 single phase, three wires
Two phase AC system
 two-phase, three wires
 two-phase, four wires
Three phase AC system
 three-phase, three wires
 three-phase, four wires
DC system
 DC two wires
 DC two wires with midpoint earthed
 DC three wires

8. Electric power transmission can also be carried out
using underground cables.
But, construction of an underground transmission line generally
costs 4 to 10 times than an equivalent distance overhead line.
However, it is to be noted that, the cost of constructing
underground transmission lines highly depends upon the local
environment. Also, the cost of conductor material required is
one of the most considerable charges in a transmission
system.
Since conductor cost is a major part of the total cost, it has to be
taken into consideration while designing.
The choice of transmission system is made by keeping in mind
various factors such as reliability, efficiency and economy.
Usually, overhead transmission system is used.

9. MAIN ELEMENTS OF A TRANSMISSION LINE
Due to the economic considerations, three-phase three-wire
overhead system is widely used for electric power
transmission. Following are the main elements of a typical
power system.
Conductors: three for a single circuit line and six for a double
circuit line. Conductors must be of proper size (i.e. cross-
sectional area). This depends upon its current capacity.
Usually, ACSR (Aluminium-core Steel-reinforced) conductors
are used.
Transformers: Step-up transformers are used for stepping up
the voltage level and step-down transformers are used for
stepping it down. Transformers permit power to be transmitted
at higher efficiency.

10. Line insulators: to mechanically support the line
conductors while electrically isolating them from the
support towers.
Support towers: to support the line conductors
suspending in the air overhead.
Protective devices: to protect the transmission
system and to ensure reliable operation. These
include ground wires, lightening arrestors, circuit
breakers, relays etc.
Voltage regulators: to keep the voltage within
permissible limits at the receiving end.